1. Field of the Invention
The present invention relates generally to a vehicle speed control system for a motor vehicle having an automatic transmission control system and an automatic speed control system.
2. Description of the Prior Art
Typical conventional vehicle speed control systems include the following:
A device for controlling a lockup clutch to disengage according to the vehicle velocity at the time the automatic speed control system is controlling the vehicle velocity because of antihunting of the vehicle, is shown in the Japanese Laid-open Patent Application Publication No. SHO61-218442, dated Sept. 27, 1986.
A device for down-shifting the automatic transmission gear according to the acceleration of the vehicle during the time the automatic speed control system is controlling the vehicle velocity is shown in Japanese Laid-open Patent Application Publication No. SHO61-238523, dated Oct. 23, 1986. This down-shifting occurs because the operator manually accelerates the vehicle, and is irrespective of the resistance to vehicle movement.
A device for down-shifting the automatic transmission gear to decelerate the vehicle while the automatic speed control system is operating is shown in Japanese Laid-open Patent Application Publication No. SHO61-238531, dated Oct. 23, 1986. This down-shifting occurs to prevent increased velocity in combination with the closing of the throttle by the automatic throttle control system.
A device for down-shifting the automatic transmission gear when the throttle is at its maximum opening and the vehicle velocity is decreasing while the automatic speed control system is operating is shown in the Japanese Laid-open Patent Application Publication No. SHO60-1031, dated Jan. 7, 1985. This down-shifting occurs because of antihunting of the vehicle.
These devices are all intended to increase the comfort for vehicle occupants. However, when the automatic transmission control changes gears while the automatic speed control system is on, the speed stage may shift at the same time the automatic speed control starts. This can occur for example, when the switch for the automatic speed control is pushed. This may be uncomfortable, because the operator may not expect the gear change at the time the speed control is activated.
Further, if the automatic transmission becomes defective, a change of speed stage caused by movement of the throttle valve of the automatic speed control system may result in an unexpected and abrupt shift between speed stages, causing an unexpected shock to vehicle occupants.
When the automatic speed control system is controlling the vehicle velocity, both the avoidance of hunting and a relatively rapid return to the set vehicle velocity are necessary. Further, shocks resulting from shifts of the speed stage and operation of the throttle valve should be minimized.